Cyclic esters of the general formula ##STR1## wherein R.sup.1 and R.sup.2 can be independently hydrogen or an aliphatic hydrocarbon having from 1 to about 6 carbon atoms, are a useful class of compounds that can be polymerized into high molecular weight polymeric plastic materials particularly useful in medical applications such as wound closure devices, orthopedic implants, and controlled release drug carriers.
In the past, these cyclic esters have been prepared by first making a brittle, polymeric form of the .alpha.-hydroxy acid corresponding to the cyclic ester. For example, if the desired product was glycolide, glycolic acid would be converted to a brittle polymeric form of polyglycolic acid. The polymeric material would then be ground to a fine powder and added slowly to a heated, evacuated vessel in which it would be thoroughly depolymerized to a crude material which had to be subjected to an extensive and costly purification operation. This process suffered from excessive tar formation, low yields, and slow production rates. One attempt to improve upon that thermal cracking process and to prepare relatively pure glycolide is described in U.S. Pat. No. 3,763,190 to Donald L. Ross. That process required first making a salt of an O-haloacetylglycolic acid and then heating the salt to a sufficient temperature to effect ring closure. Mineral salts had to be removed and the resulting glycolide separated and purified by sublimation.
The process of the present invention is a thermal cracking process that does not involve formation of a salt of a halogenated .alpha.-hydroxy acid. The present invention is distinguished from past thermal cracking processes such as the one described above in that excessive tar formation is avoided, yields are substantially higher, reaction rates are much faster, and as a result of low tar formation, the process can be run continuously, as well as batchwise.
The process of this invention involves first making a block polymer comprising a thermally stable polyether core with .alpha.-hydroxy acid or its ester polymerized onto the ends of the core. Upon heating under vacuum conditions, the chain ends are thermally degraded to form a cyclic ester which can be condensed under vacuum leaving behind any water or alcohol and tar formed in the residual thermally stable polyether which can be recycled to make added copolymer with fresh .alpha.-hydroxy acid or its ester. The copolymer can then be thermally decomposed to evolve added cyclic esters. The crude cyclic ester can then be solvent extracted and recrystallized to form a highly pure product.
The product of the present invention is purer than the product of other heretofore known processes.